1. Field of the Invention
The present invention relates to a light emitting device with a light emitting element that emits fluorescent light or phosphorescent light upon application of electric field to a pair of electrodes of the element which sandwich a film containing an organic compound (the film is hereinafter referred to as organic compound layer), and to a method of manufacturing the light emitting device. In the present invention, the term tight emitting device includes to an image display device that uses a light emitting element Also, the following modules are all include in the definition of the light emitting device: a module obtained by attaching to a light emitting element a connector such as an FPC (flexible printed circuit), a TAB (tape automated bonding) tape, or a TCP (tape carrier package); a module in which a printed wiring board is provided at an end of the TAB tape or the TCP; and a module in which an IC (integrated circuit) is directly mounted to a light emitting element by the COG (chip on glass) method.
2. Description of the Related Art
Light emitting elements, which employ organic compounds as light emitting member and are characterized by their thinness and light weight, fast response, and direct current low voltage driving, are expected to develop into next-generation flat panel displays. Among display devices, ones having light emitting elements arranged to form a matrix shape are considered to be particularly superior to the conventional liquid crystal display devices for their wide viewing angle and excellent visibility.
It is said that light emitting elements emit light through the following mechanism: a voltage is applied between a pair of electrodes that sandwich an organic compound layer, electrons injected from the cathode and holes injected from the anode are re-combined at the luminescent center of the organic compound layer to form molecular excitons, and the molecular excitons return to the base state while releasing energy to cause the light emitting element to emit light. Known as excitation states are singlet excitation and triplet excitation, and it is considered that luminescence can be conducted through either one of those excitation states.
Such light emitting devices having light emitting elements arranged to form a matrix can employ passive matrix driving (simple matrix light emitting devices), active matrix driving (active matrix light emitting devices), or other driving methods. However, if the pixel density is large, active matrix light emitting devices in which each pixel (or each dot) has a switch are considered as advantageous because they can be driven with low voltage.
Organic compounds for forming an organic compound layer (strictly speaking, light emitting layer), which is the center of a light emitting element, are classified into low molecular weight materials and polymeric (polymer) materials. Both types of materials are being studied but polymeric materials are the ones that are attracting attention because they are easier to handle and have higher heat resistance than low molecular weight materials.
Known methods for forming these organic compounds into films are, for example, ink jet, evaporation, and spin coating. The ink jet method is particularly well known as a method that allows a light emitting device to display a full-color image using a polymeric material.
However, special devices such as a high-accuracy stage, an automatic alignment mechanism, and an ink head are necessary when a full-color flat panel display that emits red light, green light, and blue light is to he manufactured by the ink jet method.
In addition, the ink jet method has the problem of ‘off-course flight’; the ink jet method sends a jet of polymeric material into the air and, if the distance between the application target surface and the nozzle of the ink jet head is not set appropriately, the shot misses the target.
Furthermore, the ink jet method ejects ink containing an organic compound from a nozzle and a problem of a clogged nozzle when the ink has high viscosity occurs. If the viscosity of the ink is lowered to counter this problem, the concentration of the organic compound contained in the ink is lowered as well and the film formed from this ink has a smaller thickness than the film that is formed under the same condition from the ink that is not thinned. Accordingly, the function thereof cannot be fully exerted or the number of times the thinned ink has to be ejected increases.
Alternatively, the problem of clogged nozzle can be solved by increasing the diameter of the nozzle. However, it is impossible to perform high-precision patterning with a nozzle having large diameter.
These problems of off-course flight and clogged nozzle are detailed in JP 11-54270 A mentioned above.
Moreover, when a film is formed by patterning using the ink jet method, it is necessary to form a tall bank on the surface for forming the film and to conduct surface treatment in order to raise the accuracy of the film formation. Specifically, the substrate is exposed to plasma so that only the bank surface is ink-proof and the ink adheres to a desired position.
As described above, the ink jet method requires the use of the special ink jet devices, optimization of ink, and treatment of the surface to which the ink is to he applied. Therefore, the ink jet method is not the optimum film formation method, nor an easy method.